Although it may help locate game it should be relied on over good tracking skills. Some of the local bowhunting assoc who have access to "touchy" hunting grounds in residential areas (Lots of tree huggers in my woods) require that these be used to "ensure" all game is found. Personally I think it's a load of crap but it's their rules.

My personal concern about using one of these is that it will effect arrow flight. When I was younger I did some bow fishing for suckers and found the string to slow down my arrow and make it less accurate then it would otherwise be.